Spot-size converter for optical mode conversion and coupling between two waveguides

ABSTRACT

A spot-size converter for coupling light between first and second waveguides respectively supporting first and second propagation modes having substantially different dimensions is provided. The spot-size converter includes a lower and an upper waveguiding structure each characterized by an effective refractive index. The lower waveguiding structure is coupled to the first waveguide to receive light therefrom or transmit light thereto in the first propagation mode. The upper waveguiding structure is coupled to the second waveguide to transmit light thereto or receive light therefrom in the second propagation mode. The upper waveguiding structure includes a plurality of longitudinally extending high-index elements arranged in multiple vertically spaced rows. At least part of the upper waveguiding structure extends at least partially over the lower waveguiding structure so as to define a coupling region wherein an effective refractive index crossing occurs, thereby enabling evanescent coupling of light between the lower and upper waveguiding structures.

RELATED PATENT APPLICATIONS

This patent application incorporates by reference, in their entirety, and claims priority to U.S. Provisional Patent Application No. 61/946,068 filed Feb. 28, 2014 and to U.S. Provisional Patent Application No. 61/974,140 filed Apr. 2, 2014.

TECHNICAL FIELD

The general technical field relates to integrated photonic waveguides and, in particular, to spot-size converters providing mode conversion between two waveguides supporting propagation modes of different dimensions.

BACKGROUND

Silicon-based integrated photonics allows for the formation of compact lightwave circuits and is compatible with silicon electronics and standard complementary metal-oxide-semiconductor (CMOS) fabrication methods. The high refractive index contrast between the silicon core and the surrounding silica (SiO₂) enables the propagation of highly confined optical modes, which allows scaling integrated photonic waveguides down to submicron level. A result of this high refractive index contrast is that silicon-based integrated photonic waveguides exhibit large modal structural birefringence between the two orthogonal transverse electric (TE) and transverse magnetic (TM) fundamental modes of the guided light. Hence, in a conventional 450-nm-wide by 220-nm-high silicon waveguide, the TE and TM modes generally have effective refractive indices that differ substantially, for example around 2.35 and 1.74 respectively. Due to its greater tolerance to bending losses, the TE mode is preferred in most practical applications.

An existing approach to couple light into a silicon photonic chip involves an inverted taper. For example, the width of a 450-nm-wide and 220-nm-high waveguide (single-mode operation at 1550 nm) can be longitudinally tapered down to a tip having a width of about 150 nm. As light propagates toward the tip of the taper, the mode size increases while the effective mode index decreases. For example, in the case of a TE mode propagating in a 220-nm-high silicon waveguide whose width is longitudinally tapered from 450 nm down to 150 nm, the modal index gradually decreases from 2.35 to 1.46 while the mode size increases up to about 2 to 3 μm. It is therefore possible to use such an inverted taper to couple light to another waveguide having similar lateral dimensions such as a lensed fiber or a very-high numerical-aperture fiber. However, using this approach for coupling to more common waveguides such as, for example, optical fibers with mode field diameters (MFDs) ranging from about 5 μm to 10 μm, leads to high coupling losses in practice. In a silicon-on-insulator (SOI) platform, the silicon waveguide sits on a 2- to 3-μm-thick buried oxide (BOX) layer, which itself sits on a silicon substrate having a thickness of several hundreds microns. If the mode expands outside of the inverted taper to increase to diameters greater than a few microns, it will tend to leak toward the silicon substrate which will be detrimental to the coupling efficiency to the optical fiber.

Recently, subwavelength gratings have been proposed to add flexibility in the design of waveguiding structures, such as described in a paper by Cheben [P. Cheben et al., “Refractive index engineering with subwavelength gratings for efficient microphotonic couplers and planar waveguide multiplexers”, Optics Letters, Vol. 35, Issue 15, pp. 2526-2528 (2010)]. When alternating the nature of the materials (typically silicon and silica) along the propagation axis of a waveguiding structure following a pattern having a period which is less than half the wavelength of the guided light, the overall waveguide behaves as a uniform material having an intermediate refractive index whose value lies between those of the two alternating materials. This can provide a way to engineer the refractive index of a waveguide to tailor its optical mode. However, using subwavelength gratings alone may not be sufficient to increase the mode size beyond what can be achieved with an inverted taper although it may provide a more practical way of obtaining the same result.

Accordingly, various challenges exist in the development of spot-size converters for photonic integrated circuits that can achieve efficient optical mode conversion and coupling between two dissimilar waveguides.

SUMMARY

According to an aspect, there is provided a spot-size converter for coupling light between a first waveguide and a second waveguide respectively supporting a first and a second propagation mode having substantially different dimensions, the spot-size converter extending along a longitudinal waveguiding axis and including:

-   -   a lower waveguiding structure having a first and a second end         opposed to the first end, the first end being coupled to the         first waveguide to receive light therefrom or transmit light         thereto in the first propagation mode, the lower waveguiding         structure being characterized by an effective refractive index;         and     -   an upper waveguiding structure having a first and a second end         opposed to the first end, the first end being coupled to the         second waveguide to transmit light thereto or receive light         therefrom in the second propagation mode, the upper waveguiding         structure being characterized by a second effective refractive         index and including a plurality of high-index elements extending         substantially along the waveguiding axis and arranged in         multiple vertically spaced rows, at least part of the upper         waveguiding structure extending at least partially over the         lower waveguiding structure so as to define a coupling region         wherein a crossing of the first and second effective refractive         indices occurs, thereby enabling evanescent coupling of light         between the lower and upper waveguiding structures.

According to another aspect, there is provided a spot-size converter for coupling light between a first waveguide and a second waveguide respectively supporting a first and a second propagation mode having substantially different dimensions, the spot-size converter extending along a longitudinal waveguiding axis and including:

-   -   a single lower waveguide element having a first and a second end         opposed to the first end, the first end being coupled to the         first waveguide to receive light therefrom or transmit light         thereto in the first propagation mode, the lower waveguide         element being characterized by a longitudinally varying first         effective refractive index that decreases toward the second end;         and     -   an upper waveguiding structure having a first and a second end         opposed to the first end, the first end being coupled to the         second waveguide to transmit light thereto or receive light         therefrom in the second propagation mode, the upper waveguiding         structure being characterized by a second effective refractive         index and including a plurality of high-index elements extending         substantially along the waveguiding axis and forming an array of         rows and columns in a plane transverse to the waveguiding axis,         at least part of the upper waveguiding structure extending at         least partially over the lower waveguide element so as to define         a coupling region wherein a crossing of the first and second         effective refractive indices occurs, thereby enabling evanescent         coupling of light between the lower waveguide element and the         upper waveguiding structure.

According to another aspect, there is provided a silicon photonic spot-size converter for coupling light between a first waveguide and a second waveguide respectively supporting a first and a second propagation mode having substantially different dimensions, the spot-size converter extending along a longitudinal waveguiding axis and including:

-   -   a silicon substrate;     -   an insulating layer formed on the silicon substrate;     -   a silicon core formed on the insulating layer, the silicon core         having a first end and a second end opposed to the first end,         the first end being coupled to the first waveguide to receive         light therefrom or transmit light thereto in the first         propagation mode, the silicon core being characterized by a         longitudinally varying first effective refractive index that         decreases toward the second end; and     -   a plurality of high-index rods extending substantially along the         waveguiding axis and forming an array of rows and columns in a         plane transverse to the waveguiding axis, each high-index rod         having a first end and a second end opposed to the first end,         the first end being coupled to the second waveguide to transmit         light thereto or receive light therefrom in the second         propagation mode, the plurality of high-index rods being         collectively characterized by a second effective refractive         index, at least part of the upper waveguiding structure         extending at least partially over the silicon core so as to         define a coupling region wherein a crossing of the first and         second effective refractive indices occurs, thereby enabling         evanescent coupling of light between the silicon core and the         plurality of high-index rods.

Other features and advantages of the embodiments of the present invention will be better understood upon reading of preferred embodiments thereof with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawings will be provided by the Office upon request and payment of the necessary fee.

FIGS. 1A and 1B are representations of the mode profiles (real part of electrical field) of the TE and TM propagation modes at 1550 nm of a 450 nm×220 nm strip silicon waveguide, obtained using a mode solver.

FIG. 2 is a representation of the mode profile of the real part of the electrical field of the fundamental mode of a SMF-28® single-mode optical fiber manufactured by Corning Incorporated, obtained using a mode solver.

FIG. 3 is a schematic perspective view of a spot-size converter for coupling light between two dissimilar waveguides, in accordance with an embodiment.

FIG. 4 is a front view of the spot-size converter of FIG. 3, taken along a plane perpendicular to the waveguiding axis of the spot-size converter.

FIG. 5 is a top view of the spot-size converter of FIG. 3.

FIG. 6 is a side view of the spot-size converter of FIG. 3.

FIG. 7 depicts numerical calculations of effective refractive index curves for the TE mode plotted as a function of position along the waveguiding axis of the spot-size converter of FIG. 3. The solid and dashed curves represent respectively the effective refractive indices of the lower and upper waveguiding structures considered separately. The dotted curve represents the effective refractive index of the spot-size converter as a whole resulting from the coupling of the lower and upper waveguiding structures. The origin along the horizontal axis corresponds to the first end of the lower waveguiding structure of the spot-size converter. FIG. 7A is an enlargement of a portion of FIG. 7 that better illustrates the crossing of the effective refractive indices of the lower and upper waveguiding structures in the coupling region.

FIGS. 8A and 8B are schematic front views of a spot-size converter for coupling light between two dissimilar waveguides, in accordance with two other embodiments, illustrating different arrangements of the high-index elements of the upper waveguiding structure.

FIG. 9 is a schematic perspective view of a spot-size converter for coupling light between two dissimilar waveguides, in accordance with another embodiment, wherein the plurality of high-index elements consists of a stack of vertically spaced high-index sheets.

FIGS. 10A to 10C are schematic perspective views of a spot-size converter for coupling light between two dissimilar waveguides, in accordance with three other embodiments. In FIG. 10A, the lower and upper waveguiding structures have respectively a uniform cross-sectional profile and a longitudinally tapered profile. In FIG. 10B, both the lower and upper waveguiding structures have longitudinally tapered profiles. In FIG. 10C, the high-index elements of the upper waveguiding structure have longitudinally tapered profiles.

FIG. 11A is a schematic front view of a spot-size converter for coupling light between two dissimilar waveguides, in accordance with another embodiment, where the silicon substrate has been etched below the waveguiding region of the spot-size converter. FIG. 11B is a variant of the embodiment of FIG. 11A where the etched region has been filled with non-light-absorbing material.

FIGS. 12A and 12B are schematic top views of other exemplary configurations of a lower waveguiding structure of a spot-size converter for coupling light between two dissimilar waveguides, in accordance with two other embodiments, both configurations of the lower waveguiding structure including a subwavelength composite portion that defines a subwavelength pattern having a characteristic size which is less than half a wavelength of light when propagating therein.

FIG. 13 is a top view of an exemplary configuration of a lower waveguiding structure of a spot-size converter for coupling light between two dissimilar waveguides, in accordance with another embodiment, the lower waveguiding structure including a plurality of high-index elements lying in a common horizontal plane.

FIG. 14A depicts the index profile of a multi-sheet upper waveguiding structure formed of four vertically spaced high-index sheets members made of a material having a refractive index of 1.58 and embedded in a low-index surrounding medium. FIGS. 14B and 14C are simulated representations of the cross-sectional mode profiles of the TE and TM propagation modes of the upper waveguiding structure of FIG. 14A.

FIGS. 15A, 15B and 15C are respectively schematic front, top and side views of a spot-size converter for coupling light between two dissimilar waveguides, in accordance with another embodiment. The lower and upper waveguiding structures of the spot-size converter are vertically evanescently coupled to each other and respectively consist of three coplanar waveguide elements and a stack of four vertically spaced high-index sheets, both in a low-index surrounding medium.

FIGS. 16A to 16I are the mode profiles calculated with a mode solver (intensity distribution in logarithmic scale) of the TE mode of the spot-size converter of FIG. 3 at different locations along the waveguiding axis going from the lower to the upper waveguiding structure up to the second waveguide.

FIGS. 17A to 17C depict the TE and TM effective refractive indices (TE mode: solid curve; TM mode: dashed curve) of tapers tapering down from 300 nm to 50 nm in width and having different uniform thicknesses (FIG. 17A: 150 nm; FIG. 17B: 180 nm; and FIG. 17A: 220 nm).

FIG. 18A is a schematic perspective view of a lower waveguiding structure of a spot-size converter for coupling light between two dissimilar waveguides, in accordance with another embodiment, wherein the lower waveguiding structure is configured as a two-level taper including a 50-μm-long single-thickness portion and a 100-μm-long double-thickness portion. FIGS. 18B and 18C are variants of the two-level taper of FIG. 18A wherein the top layer (FIG. 18B) and both the bottom and top layers (FIGS. 18B and 18C) includes a subwavelength composite portion that defines a subwavelength pattern similar to those depicted in FIGS. 12A and 12B.

FIG. 19 depicts the TE and TM effective refractive indices (TE mode: solid curve; TM mode: dashed curve) of the lower waveguiding structure of FIG. 18A plotted as a function of its length.

FIG. 20 is a schematic side view of an exemplary butt-coupling approach between a spot-size converter and an optical fiber.

FIG. 21 is a schematic side view of an exemplary evanescent coupling approach between a spot-size converter and a side polished optical fiber.

FIG. 22 illustrates an exemplary configuration of an upper waveguiding structure of a spot-size converter for coupling light between two dissimilar waveguides, in accordance with another embodiment, wherein the upper waveguiding structure has a longitudinally varying cross-sectional profile. It is noted that the rows of high-index sheets are slightly shifted laterally from one another to better illustrate the configuration of the upper waveguiding structure.

FIG. 23 is a side view of an exemplary evanescent coupling approach between a spot-size converter and an optical fiber polished at a blazing angle.

FIGS. 24A and 24B are respectively schematic side and top views of a spot-size converter for coupling light between two dissimilar waveguides, in accordance with another embodiment, wherein the rows of high-index elements of the upper waveguiding structure are staggered along the waveguide axis. In FIG. 24B, the rows of high-index sheets are slightly shifted laterally from one another to better illustrate the configuration of the upper waveguiding structure.

FIG. 25 illustrates a variant of the spatially varying multi-sheet upper waveguiding structure FIGS. 24A and 24B, according to which the high-index sheets are tapered at their end overlying the lower waveguiding structure. It is noted that the rows of high-index sheets are slightly shifted laterally from one another to better illustrate the configuration of the upper waveguiding structure.

FIG. 26 is a schematic side view of an exemplary configuration of an upper waveguiding structure of a spot-size converter for coupling light between two dissimilar waveguides, in accordance with another embodiment, wherein the rows of high-index elements of the upper waveguiding structure are staggered along the waveguide axis to form a curved overall waveguiding structure in a vertical plane.

FIG. 27 is a schematic top view of a spot-size converter for coupling light between two dissimilar waveguides, in accordance with an embodiment, wherein the lower waveguiding structure includes two tapered waveguide elements tailored to couple to preferentially the TE and TM modes separately. It is noted that the rows of high-index sheets are slightly shifted laterally from one another to better illustrate the configuration of the upper waveguiding structure.

FIG. 28 is a schematic perspective view of a spot-size converter for coupling light between two dissimilar waveguides, in accordance with another embodiment.

FIGS. 29A and 29B are respectively schematic perspective and top views of a spot-size converter for coupling light between two dissimilar waveguides, in accordance with another embodiment.

FIG. 30 is a graph illustrating experimentally measured variation of insertion loss in dB versus wavelength in the C-band for the TE and TM modes of the spot-size converter of FIG. 8A. The spot-size converter is used for coupling light between a strip silicon waveguide and a second waveguide consisting of a 6.6-μm MFD optical fiber fusion-spliced to a SMF-28® single-mode optical fiber (Corning Incorporated). The total coupling loss includes propagation and evanescent coupling losses in the spot-size converter, the loss contribution from modal mismatch between the spot-size converter and the second waveguide as well as the fusion splice loss between the second waveguide and the SMF-28® optical fiber.

DETAILED DESCRIPTION

In the following description, similar features in the drawings have been given similar reference numerals, and, in order to not unduly encumber the figures, some elements may not be indicated on some figures if they were already identified in preceding figures. It should also be understood herein that the elements of the drawings are not necessarily depicted to scale, since emphasis is placed upon clearly illustrating the elements and structures of the present embodiments.

General Overview—Spot-Size Converters

The present description generally relates to spot-size converters for coupling light between a first waveguide supporting a first propagation mode and a second waveguide supporting a second propagation mode, where the first and second propagation modes having substantially different dimensions.

As discussed in greater detail below, the spot-size converters described herein generally include a lower waveguiding structure coupled to the first waveguide and an upper waveguiding structure coupled to the second waveguide. The upper waveguiding structure, which is formed of a plurality of high-index elements arranged in multiple vertically spaced rows, extends at least partially over the lower waveguiding structure to define a coupling region in which a vertical evanescent coupling of light is established between the lower and upper waveguiding structures.

As used herein, the term “spot-size converter” refers to a transitional light-guiding and transforming element configured to change or convert the mode size of a light beam to be transmitted between two waveguides carrying optical modes of different dimensions. Spot-size converters according to embodiments of the invention may be useful in silicon-based photonic integrated circuits or other high-index-contrast photonics applications where it is desired to match the spot size of light in a silicon-based or another high-index-contrast photonic waveguide to the spot size of light in another, usually larger, optical component, or vice versa.

As used herein, the term “waveguide” refers to a structure that is designed to confine and direct the propagation of light such that the electromagnetic energy of the one or more guided modes supported by the structure remains substantially confined therein. Of course, as one skilled in the art will readily understand, guided modes generally present an evanescent field that extends partially outside of the waveguide.

As used herein, the terms “light” and “optical” are understood to refer to electromagnetic radiation in any appropriate region of electromagnetic spectrum, and are not limited to visible light but also generally include the microwave, infrared and ultraviolet ranges. For example, in embodiments of the invention for use in the telecommunications industry, the term “light” and “optical” may encompass electromagnetic radiation with a wavelength ranging from about 1260 nm to 1675 nm, including the O-band, C-band and L-band transmission windows for telecommunication applications.

As used herein, the term “high-index” when referring to an element of the upper or lower waveguiding structure of the spot-size converter refers to an element (e.g., a rod or a sheet) having an refractive index which is sufficiently higher than a refractive index of the medium surrounding this element for the upper or lower waveguiding structure to behave as an overall waveguiding structure. In other words, the term “high-index” is to be interpreted as a relative term indicating that the refractive index of the high-index element is higher than that of its surrounding environment, and not as an absolute term implying a specific numerical range of refractive index. By way of example only, and without limitation, in an implementation where a high-index element is surrounded by silica having a refractive index of about 1.44 at 1550 nm, the material forming the high-index element may have a refractive index higher than about 1.50 at 1550 nm.

In some embodiments, the spot-size converter may be based on silicon photonic device and implemented on an SOI platform. As known in the art, SOI technology refers to an integrated circuit technology that uses a layered silicon-insulator-silicon substrate rather than conventional silicon substrates. The thin layer of silicon formed on top of the insulating layer, typically silica, can be patterned to define one or more waveguides, as required by a given circuit design. Compared to technologies for photonic integration based on other combinations of materials, SOI is particularly attractive as it can provide very compact optical circuits. Such optical circuits are usually made by etching part of a thin layer of silicon (e.g., 220 nm thick) lying on top of a buried silicon oxide insulating layer (e.g., 1-3 μm thick), itself formed on top of a thick silicon substrate (e.g., several hundreds of μm). The provision of the buried silicon oxide insulating layer within the wafer can allow light to be highly confined in the waveguides forming the optical circuits. Of course, it is to be understood that the spot-size converters described herein are not limited to the SOI technology but may be based on various other types of materials such as, for example, silicon nitride (Si₃N₄), silicon carbide (SiC), silicon oxynitride (SiO_(x)N), silicon oxide (SiO_(x)), indium phosphide (InP), gallium arsenide (GaAs), polymers and the like.

The first and second waveguides may be embodied by various pairs of dissimilar waveguides which are to be optically coupled via the spot-size converter.

In one implementation, the first waveguide may be embodied by a submicron strip silicon waveguide, such as used on silicon photonics chips. The first waveguide may be optically connected to a first end of the spot-size converter (e.g., physically butt coupled or adiabatically coupled). For example, referring to FIGS. 1A and 1B, there are shown simulated representations of the mode profiles (real part of electrical field) of the TE and TM modes at 1550 nm of an exemplary 450-nm-wide by 220-nm-high strip silicon waveguide. In other implementations, the first waveguide may be embodied by another type of planar waveguide structure such as, for example, a rib, a ridge or a slot waveguide, which are also commonly found in silicon photonics. As used herein, the term “planar waveguide” is understood to refer to a waveguide provided in a substantially integrated configuration, such as in a planar light circuit, and including a light guiding path supported by a suitable substrate. However, while the term “planar” may suggest a flat structure, the term “planar waveguide” is also meant to encompass structures made of a plurality of flat layers stacked on a substrate, one of these layers defining the light-guiding path, as for SOI-based structures.

The second waveguide may be embodied by various structures carrying an optical mode of different dimensions than the optical mode carried by the first waveguide it is coupled to through the spot-size converter. The term “dimensions” when referring to an optical mode supported by the first or second waveguide is intended to refer to at least one parameter (e.g., the mode size or shape) characterizing the cross-sectional profile of the mode. More specifically, the cross-sectional profile of a mode refers to the spatial profile of the mode along a plane transverse to the propagation direction. It is to be noted that depending on the application, the first and second modes respectively supported by the first and second waveguide may be different in size, shape or both.

In some implementations, the second waveguide may be a larger SOI structure or the like or an optical fiber used to couple light in or out of an SOI chip. Optical fibers usually have light guiding cores of dimensions of the order of 5-10 μm, which are much greater than those of typical submicron silicon photonic waveguides. By way of example, FIG. 2 illustrates the 1550 nm cross-sectional mode profile of the fundamental mode of a SMF-28® single-mode optical fiber (Corning Incorporated). By comparison to FIG. 1A, it will be observed that the optical fiber mode depicted in FIG. 2 is about 400 times larger than the guided mode supported by a typical submicron silicon waveguide.

In other implementations, the second waveguide may be a strip waveguide, a rib waveguide, a slot waveguide, a ridge waveguide or another planar waveguide implemented in a silicon photonics chip, an indium phosphide chip or another chip design. Depending on the application, the second waveguide can be optically coupled to one end of the spot-size converter (e.g., physically butt coupled) or to the top of the spot-size converter to facilitate evanescent light coupling. In further implementations, the coupling between the second waveguide and the spot-size converter may be a free-space coupling to couple a laser beam from a laser source into the first waveguide or to couple a free-space beam into the first waveguide, or to generate a free-space beam out of the spot-size converter, via a lens or appropriate optics, if necessary.

It will be readily understood that the designation of “first” and “second” in reference to the waveguides coupled through the spot-size converter is used merely for clarity and convenience, namely to differentiate the two waveguides. In particular, the use of the terms “first” and “second” is not meant to convey or imply a particular light propagation direction between the two waveguides. Accordingly, light may travel from the first to the second waveguide or from the second to the first waveguide, without departing from the scope of the present invention.

Exemplary Embodiment of a Spot-Size Converter

Referring to FIGS. 3 to 6, there is illustrated an exemplary embodiment of a spot-size converter 100 for coupling light between a first waveguide 102 and a second waveguide 104 respectively supporting a first and a second propagation mode. The first and second waveguides 102, 104, as well as the respective propagation modes carried thereby, have substantially different dimensions. The spot-size convertor 100 extends between a first end 106 a and an opposite second end 106 b along a longitudinal waveguiding axis 108 so as to define a light path between the first and second waveguides 102, 104.

In this embodiment, the first waveguide 102 is a conventional SOI-based strip waveguide having a width of 450 nm and a height of 220 nm. The first waveguide 102 is optically connected to the first end 106 a of the spot-size converter 100 (physically butt coupled in this embodiment). In this configuration, the first waveguide 102 is found to be single-mode for both TE and TM modes. Of course, this configuration for the first waveguide 102 is provided by way of example only, since various other structures, geometrical dimensions and coupling arrangement to the spot-size converter could be used. For example, in other embodiments, the first waveguide 102 may be embodied by any other appropriate types of single-mode or multimode planar waveguides including, but not limited to, a rib waveguide, a slot waveguide and a ridge waveguide.

The second waveguide 104 is embodied by a single-mode optical fiber which is optically connected to the second end 106 b of the spot-size converter 100 (physically butt coupled in this embodiment). The optical fiber includes a fiber core 110 having an MFD of about 6.6 μm and defining a light-guiding path of the second waveguide 104. The fiber core 110 is surrounded by a fiber cladding 112. It will be understood that embodiments of the present invention are not limited to a particular type or size of optical fiber. In particular, the optical fiber may be made of plastic, glass, quartz, silica or the like, and may include, without being limited to single-mode fibers, multimode fibers, polarization-maintaining fibers, microstructured fibers, multicore fibers, single-cladding fibers, multi-cladding fibers, doped fibers, high or ultra high numerical aperture fibers, fiber arrays, or any other non-typical or customized fibers.

The spot-size converter 100 in FIGS. 3 to 6 is an SOI-based spot-size converter including a silicon substrate 114, an insulating layer 116 made of silica or another suitable material deposited on the silicon substrate 114, and a waveguiding region 118 provided on top of the insulating layer 116 and embedded in a cladding structure 120. As discussed further below, the cladding structure 120 may be formed of one or more cladding layers, each of which can be made of silica or another suitable material.

It is to be noted that while the insulating layer 116 and the cladding structure 120 may both be made of the same material (e.g., silica), their refractive indices may differ slightly due to the use of different deposition techniques. It is also to be noted that, in the illustrated embodiment, the spot-size converter 100 and the first waveguide 102 are formed from the same SOI wafer, so that they share the same silicon substrate 114, insulating layer 116 and cladding structure 120. As mentioned above, in other embodiments, the spot-size converter 100 need not be based on SOI technology.

The waveguiding region 118 of the spot-size converter 100 is responsible for converting the mode size of light propagating along the waveguiding axis 108 between the first and second waveguides 102, 104. The waveguiding region 118 generally includes a lower waveguiding structure 122 and an upper waveguiding structure 124. The lower waveguiding structure 122 has a first end 126 a and an opposed second end 126 b, the first end 126 a being coupled to the first waveguide 122 to receive light therefrom or transmit light thereto in the first propagation mode. Similarly, the upper waveguiding structure 124 has a first end 128 a and an opposed second end 128 b, the first end 128 a being coupled to the second waveguide 104 to receive light therefrom or transmit light thereto in the second propagation mode. Depending on the application, each of the lower and upper waveguiding structures 122, 124 can be made of various materials including, but not limited to, silicon, silicon oxynitride, silicon oxide, silicon nitride, silicon carbide, indium phosphide, gallium arsenide, a polymer or a combination thereof.

Lower Waveguiding Structure

Referring still to FIGS. 3 to 6, the lower waveguiding structure 122 may consist of a single waveguide element made of silicon and extending along the longitudinal waveguiding axis 108. The lower waveguiding structure 122 is formed on the insulating layer 116 of the SOI wafer and is patterned from the same thin silicon layer as the first waveguide 102. In the illustrated embodiment, the lower waveguiding structure 122 is single-mode for both TE and TM modes, but this need not be the case in other embodiments.

In some implementations, the lower waveguiding structure 122 can include a longitudinally tapered portion 130 tapering down toward the second end 126 b thereof. It is noted that in the embodiment of FIGS. 3 to 6, the longitudinally tapered portion 130 of the lower waveguiding structure 122 extends over its entire length, but this need not be the case in other embodiments. More specifically, in FIGS. 3 to 6, the lower waveguiding structure 122 has a length of 500 μm, a constant height of 220 nm and a width that tapers from 450 nm at the first end 126 a to 120 nm at the second end 126 b. In some implementations, the width of the lower waveguiding structure 122 at the second end 126 b is ideally as small as possible and is generally limited by the manufacturing process.

It is noted that in some implementations, the lower waveguiding may have a first longitudinally tapered portion tapering from the first end down to an intermediate location and a second longitudinally tapered portion tapering from the intermediate location down to the second end, where the profile (e.g., the slope) of the first longitudinally tapered portion is different (e.g., steeper) than the profile (e.g., the shape) of the second longitudinally tapered portion. Such an arrangement may contribute to reduce the overall length of the lower waveguiding structure.

As use herein, the terms “length” and variants thereof are used to refer to a dimension of a waveguide or waveguiding structure in the light propagation direction and along a longitudinal axis. The terms “width” and variants thereof are herein to refer to a dimension of a waveguide perpendicular to the light propagation and parallel the conventional plane or surface of the substrate on which the waveguide is formed. The terms “height” and variants thereof refer to a dimension of a waveguide perpendicular to both the length and width and also to the conventional plane or surface of the substrate. In some instances, the term “height” may be used substituted by the term “thickness”. In this regard, it is noted that throughout the present description, the terms “vertical” and variants thereof refer to a direction perpendicular to a plane parallel to the conventional plane or surface of the substrate, that is, along the “height” (or “thickness”) direction. Likewise, the terms “horizontal” and variants thereof are used to refer to directions lying in a plane which is perpendicular to the vertical direction as just defined, that is, encompassing the “width” and “length” directions.

The lower waveguiding structure 122 is characterized by an effective refractive index. As used herein, the term “effective refractive index”, or simply “effective index”, refers to a property of a waveguide that quantifies the decrease in the local phase velocity of light propagating in the waveguide compared to its phase velocity in vacuum. The effective refractive index represents the refractive index of a uniform material in which light would propagate with the same phase velocity as in the waveguide. As known in the art, the effective refractive index of a waveguide depends not only on its material properties, on the temperature and on the wavelength of light propagating therein, but also on its geometrical parameters (e.g., its cross-sectional profile) and, for multimode waveguides, on the mode in which light propagates. For this reason, the effective refractive index is also referred to as a “modal index”. Additionally, the effective refractive index of a waveguide can vary along the propagation direction, as a result, for example, of a longitudinally varying cross-sectional profile. It is to be noted that in the present description, the effective refractive index values are given at 1550 nm, unless stated otherwise.

Referring still to FIGS. 3 to 6, it will be understood that in order to ensure good mode overlap between the lower waveguiding structure 122 and the first waveguide 102, it is desirable that the mode size and effective refractive index of the lower waveguiding structure 122 at the first end 126 a thereof substantially coincide with those of the first propagation mode carried in the first waveguide 102. For example, for the tapered lower waveguiding structure 122 illustrated in FIGS. 3 to 6, the effective refractive index for the single TE mode varies from about 2.35 at the first end 126 a (which matches the value of the effective refractive index of the first waveguide 102) to about 1.46 at the second end 126 b, as depicted by the solid curve in FIG. 7 where the origin along the horizontal axis corresponds to the first end of the lower waveguiding structure. It will be understood that the narrower the tip of the taper, the closer the effective refractive index at the tip will be to the refractive index of the low-index medium surrounding the taper, which corresponds to the cladding structure 120 in FIGS. 3 to 6.

In the embodiment of FIGS. 3 to 6, as the lower waveguiding structure 122 tapers down toward the second end 126 b, the mode guided in the lower waveguiding structure 122 becomes less confined so as to gradually expand in the cladding structure 120. In the illustrated embodiment, the mode evolution of light propagating along the lower waveguiding structure 122 is preferably adiabatic, or close to adiabatic. As known in the field of optical waveguides, the term “adiabatic” is intended to mean that variations in the cross-sectional profile of a waveguide are sufficiently slow and smooth to render coupling to other modes and radiative losses negligible or below some operationally acceptable level. As will be discussed below, the provision of an adiabatically tapering lower waveguiding structure 122 can favor and enhance the coupling of light between the lower and upper waveguiding structures 122, 124.

It is to be understood that the configuration for the lower waveguiding structure 122 illustrated in FIGS. 3 to 6 is provided by way of example only, as various other structural arrangements, optical properties, geometrical dimensions and materials could be used in other embodiments. For example, the longitudinally tapered portion 130 in FIGS. 3 to 6 is a taper having a constant height profile, but it may alternatively or additionally have a height profile that tapers longitudinally down toward the second end 126 b of the lower waveguiding structure 122. Also, the slope of the longitudinally tapered portion 130 may be constant or not along its length (i.e., the longitudinally tapered portion 130 may form a linear or a non-linear taper), and may or may not form an adiabatic taper. More regarding other possible configurations for the lower waveguiding structure 122 will be discussed further below.

Upper Waveguiding Structure

Referring still to FIGS. 3 to 6, in the illustrated embodiment the upper waveguiding structure 124 includes a plurality of high-index elements 132 embodied by a plurality of optically coupled high-index rods extending along the waveguiding axis 108 and arranged in a two-dimensional array in a plane transverse to the waveguiding axis 108. The high-index rods can for example be made of various materials including, but not limited to, silicon, silicon oxynitride, silicon oxide, silicon nitride, silicon carbide, indium phosphide, gallium arsenide, a polymer, a combination thereof or other suitable materials.

The multi-rod array is embedded in the cladding structure 120 and includes, by way of example, three vertically spaced rows 134 a to 134 c and four horizontally spaced columns 136 a to 136 d of high-index rods. The high-index elements 132 can have a height of about 50 to 500 nm, a width of about 100 to 1000 nm, a length of several hundreds of microns, and a longitudinally uniform cross-section. The vertical spacing between adjacent rows can be a few microns while the horizontal spacing between adjacent columns can be in the range of one to a few microns. Of course these dimensions are provided by way of example only and can be varied in other embodiments. It is also noted that in the illustrated embodiment, the separation between the high-index elements is substantially larger than their corresponding lateral dimensions (i.e., width and height), but this need not be the case in other embodiments.

As better depicted in FIG. 6, in the illustrated embodiment, the lowest-lying row 134 a of the plurality of high-index elements 132 is formed on the insulating layer 116 and thus lies in a same horizontal plane as the tapered lower waveguiding structure 122. It is to be noted that in implementations where the lowest-lying row 134 a of the plurality of high-index elements 132 and the lower waveguiding structure are coplanar, it may be advantageous that the high-index elements 132 in the lowest-lying row 134 a be fabricated at the same time and that they be made of same material as the lower waveguiding structure 122. Of course, any change in the optical properties of the upper waveguiding structure resulting from having different rows of high-index elements 132 made of different materials could be compensated by adjusting the geometrical parameters of the array.

Turning briefly to FIGS. 8A and 8B, in other embodiments, the lowest-lying row 134 a of high-index elements 132 could alternatively be provided slightly above or below the lower waveguiding structure 122. First, in FIG. 8A, the cladding structure 120 includes a first cladding 138 a formed over the insulating layer 116 and sufficiently thick to surround the lower waveguiding structure 122, and a second cladding 138 b formed on the first cladding 138 a and embedding the plurality of the high-index elements 132 (e.g., the array of high-index rods). It is noted that, in practice, the second cladding 138 b may itself be made of multiple sublayers, for example three in FIG. 8A. In this scenario, the high-index elements 132 in the lowest-lying row 134 a of the array are patterned on top of the first cladding 138 a and lie slightly above the lower waveguiding structure 122. It is to be noted that while the insulating layer, the first cladding 138 a and the second cladding 138 b can all be made of the same material (e.g., silica), their refraction indices may differ due to the use of different materials and/or deposition techniques. Second, in FIG. 8B, the lower waveguiding structure 122 is patterned on the insulating layer 116 and, following selective etching of the upper surface of the insulating layer 116, the high-index elements 132 in the lowest-lying row 134 a are subsequently patterned. In this scenario, the lowest-lying row 134 a of high-index elements 132 lies slightly below the lower waveguiding structure 122.

Referring back to FIGS. 3 to 6, the upper waveguiding structure 124 is also characterized by an effective refractive index. In order to ensure good mode overlap between the upper waveguiding structure 124 and the second waveguide 104, it is desirable that the mode size and the effective refractive index of the upper waveguiding structure 124 at the first end 128 a thereof substantially coincide with those of the propagation mode guided in the second waveguide 104. In particular, this means that, in some embodiments of the spot-size converter 100, it is desirable that the upper waveguiding structure 124 be able to support modes as large as those carried in standard telecommunication optical fibers with MFDs ranging from about 8 to 10 μm or in high numerical aperture (HNA) fibers with MFDs ranging from about 3 to 8 μm and fusion-spliced to standard fibers.

It will be understood that an upper waveguiding structure 124 such as in FIGS. 3 to 6, which has relatively large lateral dimensions and is formed of a plurality of high-index elements 132, can provide an efficient way to form an overall waveguiding structure capable of carrying a mode whose size and effective refractive index can match, for example, those of a standard or HNA optical fiber. It will also be understood that when the upper waveguiding structure 124 of FIGS. 3 to 6 behaves as an overall waveguiding structure, light guided therealong travels mainly in the cladding structure 120 embedding the high-index elements 132, rather than in the high-index elements 132 themselves.

As a result, the effective refractive index of the upper waveguiding structure 124 as a whole may be significantly lower than the effective refractive index of the individual high-index elements 132 and closer to the effective refractive index of a standard optical fiber or another larger waveguide embodying the second waveguide 104. For example, the effective refractive index for the single TE mode of the multi-rod upper waveguiding structure 124 of FIGS. 3 to 6 has a constant value of about 1.48 along its length, as depicted by the dashed curve in FIG. 7. It will be understood that the effective refractive index of the upper waveguiding structure 124 will generally depend on the material properties of the high-index elements 132 and low-index cladding structure 120 and on the geometry of the high-index element array.

It will be understood that the configuration for the upper waveguiding structure 124 illustrated in FIGS. 3 to 6 is provided by way of example only, as various other structural arrangements, optical properties, geometrical dimensions and materials could be used in other embodiments. For example, while the plurality of high-index elements 132 forms a two-dimensional linear array of high-index rods in FIGS. 3 to 6, this need not be the case in other embodiments as long as the plurality of high-index elements 132 are arranged in multiple vertically spaced rows and in one or more columns. Indeed, in some embodiments, the plurality of high-index elements 132 can form of a stack of vertically spaced high-index sheets, as discussed further below and illustrated in FIG. 9. Also, while in FIGS. 3 to 6 the upper waveguiding structure 124 is arranged symmetrically with respect to the waveguiding axis 108, this need not be the case in other embodiments.

Furthermore, referring to FIG. 28, in some embodiments, the high-index rods in FIGS. 3 to 6 may be formed so as to extend further above the lower waveguiding structure 122 while also gradually laterally diverging away from the waveguiding axis 108 as they get closer to the first end 126 a of the lower waveguiding structure 122. Such a configuration may provide a smoother transition between the lower and upper waveguiding structures 122, 124.

It will also be understood that the high-index elements 132 need not be configured as a linear array but may be provided at arbitrary locations that do not conform to a specific pattern. Likewise, the cross-sectional profile of the individual high-index elements 132 need not be rectangular and may vary in size and/or shape along the waveguiding axis 108. Also, while the effective refractive index of the upper waveguiding structure 124 is substantially constant along its length in the embodiment of FIGS. 3 to 6, this need not be the case in other embodiments. More regarding other possible configurations for the upper waveguiding structure 124 will be discussed further below.

Coupling Between the Lower and Upper Waveguiding Structures

It will be understood that because the first and second waveguides carry modes with substantially different effective indices and mode sizes, so do the lower and upper waveguiding structures at their respective first ends. Therefore, in order for the spot-size converter described herein to provide a smooth and efficient transition for light propagating therein, it is desirable that the lower and upper waveguiding structures establish a coupling region therebetween, where their modes overlap and crossing of their effective refractive indices occurs. Fulfilling these conditions generally implies that the lower and upper waveguiding structures are positioned sufficiently close to each other and that the effective refractive index of at least one of the lower and upper waveguiding structures varies longitudinally inside the coupling region. More regarding the coupling between the lower and upper waveguiding structures will now be described.

Referring back to FIGS. 3 to 6, the multiple vertically spaced rows 134 a to 134 c of high-index elements 132 are positioned in a manner such that at least part of the upper waveguiding structure 124 extends above the lower waveguiding structure 122. For example, in FIGS. 3 to 6, the two highest-lying rows 134 b, 134 c extend above the lower waveguiding structure 122 while the lowest-lying row 134 a is formed on a same horizontal plane as the lower waveguiding structure 122 (see FIGS. 8A and 8B introduced above for alternative configurations).

This vertical overlap of the upper waveguiding structure 124 with the lower waveguiding structure 122 defines a coupling region 140 therebetween. As used herein, the term “coupling region” refers to a region of the spot-size converter in which light propagating in lower waveguiding structure is transferred or coupled to the upper waveguiding structure, and vice versa. In the illustrated embodiment, the lower and upper waveguiding structures 122, 124 are configured such that their modes overlap and a crossing of their effective refractive indices occurs in the coupling region 140. It will be understood that these conditions can enable or promote the establishment of a vertical evanescent coupling of light between the lower and upper waveguiding structures 122, 124.

As known in the art, evanescent optical coupling between two waveguiding structures is a coupling technique in which the evanescent field associated with light propagating in one waveguiding structure is used to couple the light in the other waveguiding structure when the two waveguiding structures are brought into close enough proximity. Evanescent coupling involves a spatial overlap between the modes guided in the two waveguiding structures, that is, a coincidence or crossing of the effective refractive indices of the two waveguiding structures at some location along the waveguiding axis. It is to be noted that when two waveguiding structures are made of different materials, which is the case for the lower and upper waveguiding structures illustrated in FIGS. 3 to 6, it may be indeed advantageous to use a vertical evanescent coupling between the two waveguiding structures.

In the embodiment of FIGS. 3 to 6, the crossing of the effective refractive indices of the lower and upper waveguiding structures 122, 124 to establish a vertical evanescent coupling therebetween is achieved by longitudinally tapering the lower waveguiding structure 122 to reduce its effective refractive index from about 2.35 at the first end 126 a to about 1.46 at the second end 126 b, as depicted in FIG. 7. As mentioned above, the longitudinal tapering of the lower waveguiding structure 122 may be done adiabatically, such that the cross-sectional profile of optical mode gradually changes in a substantially lossless manner so as to correspond to the fundamental guided mode at each location along the waveguiding axis 108.

Meanwhile, the multi-rod upper waveguiding structure 124 has a uniform cross-sectional profile along the waveguiding axis 108 with an effective refractive index of about 1.48 (dashed curve in FIG. 7), which is slightly higher than the refractive index of the low-index surrounding medium (e.g., about 1.44 if the cladding structure is made of silica). For this particular configuration, there is a point in the coupling region 140 where the effective refractive indices of the upper and lower waveguiding structures considered separately coincide. Referring to FIG. 7A, which is an enlargement of a portion of FIG. 7, this crossing point is located at about 400 μm from the first end of the lower waveguiding structure and corresponds to the intersection of the solid and dashed curves.

FIGS. 7 and 7A depicts not only the longitudinal profile of the effective refractive indices of the lower and upper waveguiding structures considered separately, but also the effective refractive index of the spot-size converter as a whole resulting from the combination of the lower and upper waveguiding structures in order to create a single waveguiding structure (see the dotted curve in FIGS. 7 and 7A). The mode associated with the effective refractive index of the spot-size converter as a whole may be referred to as a “super-mode”. By considering the longitudinal profile of the super-modes at different location along the waveguiding axis, it may be seen that the super-mode corresponds to the mode of the longitudinally tapered lower waveguiding structure at the end of the spot-size converter coupled to the first waveguide (e.g., a submicron silicon waveguide) and that it corresponds to the mode of the multi-rod upper waveguiding structure at the end of the spot-size converter coupled to the second waveguide (e.g., a standard optical fiber). In other words, the super-mode propagating in the spot-size converter gradually transfers its shape from that associated with the lower to the upper waveguiding regions along the waveguiding axis, and vice versa (see also the double-sided arrow in FIG. 6). It can thus be seen that the effective indices of the modes associated with the lower and upper waveguiding structures of the spot-size converter play an important role in the strength and enhancement of the evanescent coupling.

FIGS. 16A through 16I show the TE mode intensity distribution of an embodiment of the spot-size converter at different locations over a length of 500 μm along the waveguiding axis, going from the lower to the upper waveguiding structure up to the second waveguide. In this embodiment, the lower and upper waveguiding structures are formed respectively of a single silicon waveguide element tapering down from an initial 300-nm wide by 220-nm-high cross-section (see FIG. 16A) and of a stack of three vertically spaced high-index multi-rods layers having an MFD of about 7 μm as shown in FIG. 8A. A maximum evanescent coupling loss of 0.05 dB for the TE mode is obtained over the C-band from eigenmode-expansion (EME) simulations, which is indicative of an efficient adiabatic transfer. The coupling loss due to mode mismatch between the second end of the spot-size converter (FIG. 16H) and the second waveguide (FIG. 16I) contributes to less than 0.22 dB of the 0.27 dB total loss over the C-band for both TE and TM modes. One skilled in the art will appreciate that such values are indicative of a favourably high coupling efficiency from a standard silicon strip waveguide to an optical fiber having a large mode field diameter.

It will be understood that the provision of an upper waveguiding structure formed of a plurality of vertically spaced rows of high-index elements can allow the effective refractive index and mode size of the upper waveguiding structure to be tailored and designed by appropriately selecting the material forming the high-index elements (which may differ from one high-index element to another), their individual size and shape, as well as their relative arrangement (i.e., their relative positioning and mutual separation). In other words, the provision of multiple vertically spaced rows of high-index elements can offer more degrees of freedom than upper waveguiding structures made of a single layer of elements. In particular, this configuration of the upper waveguiding structure can provide an efficient way to form an overall waveguiding structure capable of carrying a mode whose size and effective refractive index can match those of relative large second waveguides such as, for example, optical fibers with MDF ranging from about 3 to 10 μm.

Other Configurations for the Spot-Size Converter

It will be understood that the embodiment of the spot-size converter described above with reference to FIGS. 3 to 6 was provided by way of example only, and that the spot-size converter may be provided in a variety of other possible configurations. Several examples of such possible configurations will now be discussed.

In the embodiment of the spot-size converter 100 illustrated in FIGS. 3 to 6, the lower waveguiding structure 122 is longitudinally tapered while the upper waveguiding structure 124 has a uniform cross-sectional profile along its length. However, referring to FIG. 10A, in another embodiment of the spot-size converter 100, it may be the upper waveguiding structure 124 that tapers longitudinally as a whole in the coupling region 140 while the lower waveguiding structure 122 has a uniform cross-sectional profile along its length. Similarly, referring to FIG. 10B, in a further embodiment, both the lower and upper waveguiding structures 122, 124 can taper longitudinally in the coupling region 140. Turning now to FIG. 10C, in yet another embodiment, a longitudinal variation in the effective refractive index of the upper waveguiding structure 124 can be achieved not by a longitudinal tapering of the lower or upper waveguiding structures 122, 124 as a whole, but rather by a longitudinal tapering of at least one of the plurality of high-index elements 132 of the upper waveguiding structure 124 in the coupling region 140.

It is to be noted that in other implementations, a crossing of the effective refractive indices of the lower and upper waveguiding structures need not involve a longitudinal tapering of either or both of the lower and upper waveguiding structures. Hence, in some embodiments, the effective refractive index of either or both of the lower and upper waveguiding structures can vary longitudinally due to a longitudinal variation in material composition or geometrical properties without longitudinal tapering. This could be achieved, for example, in a scenario where the lower waveguiding structure would be embodied by a subwavelength grating having a uniform cross-section profile along its length but a longitudinally varying duty cycle. More regarding subwavelength structures will be discussed further below.

In a conventional SOI structure, the insulating layer is typically 1 to 3 μm thick and is deposited on top of a significantly thicker silicon substrate. In some embodiments of the spot-size converter, using an upper waveguiding structure formed as an array of elongated high-index elements (e.g., a multi-rod array) which is large enough to provide good overlap with the mode of a standard telecommunication optical fiber may be challenging due to the presence of the thick silicon substrate. This is because, in some implementations, the light propagating in the spot-size converter may tend to leak toward the silicon substrate, thus causing unwanted losses. In some embodiments, addressing or mitigating this issue could involve reducing the length of the tapered portion of the lower waveguiding structure. In other embodiments, it could be envisioned to decrease the size of the mode carried in the upper waveguiding structure so as to avoid or mitigate losses to the substrate.

Another approach could involve increasing the thickness of the SOI insulating layer. For example, while a thickness of 2 μm generally represents a standard value for the SOI insulating layer, recent publications have reported SOI insulating layers having thicknesses in the range of 3-6 μm. A further approach could involve surrounding the upper waveguiding structure with a material that has a refractive index that is slightly higher than the refractive index of the SOI insulating layer (but preferably still less than about 1.5). In such a configuration, the optical mode carried by the upper waveguiding structure would generally remain more confined, extending less into the SOI insulating layer and thus interacting less with the silicon substrate. It is to be noted that in scenarios where the material surrounding the lower waveguiding structure is different than the material embedding the upper waveguiding structure, the material surrounding the lower waveguiding structure could have a refractive index having a value between those of the SOI insulating layer and the material embedding the upper waveguiding structure. Alternatively, the mode size could be decreased in order to avoid or mitigate losses to the substrate.

Referring to FIG. 11A, in yet another approach, the silicon substrate 114 may be etched underneath the waveguiding region 118 of the spot-size converter 100, in order to help reducing light leakage toward the substrate 114 and improving coupling to an optical fiber with larger mode-field diameters. Referring to FIG. 11B, a variant of the embodiment of FIG. 11A could further involve filling the etched region with a material 142 selected to avoid or reduce light absorption or leaks, such as for example silica or an adhesive. This particular variant may be advantageous in applications where it is desired or needed that the chip implementing the spot-size converter be affixed to another surface.

The tapered structures in the embodiments of the spot-size converter described so far include tapers whose tip has a width that is ideally as small as possible, which may be difficult or not possible to fabricate in practice. In some of these embodiments, subwavelength gratings may be used to help defining a structure that is more easily manufactured and that can act as a taper having a narrower tip. For example, referring to FIGS. 12A and 12B, the lower waveguiding structure 122 of the spot-size converter may be a single waveguide provided with a subwavelength composite portion 144 that defines a subwavelength pattern.

As used herein, the term “subwavelength” refers to the fact that the size of the characteristic features or inhomogeneities (typically, corrugation periodicity) of the subwavelength pattern are markedly smaller than half of the effective wavelength of the electromagnetic signal propagating thereinside. When the effective wavelength of the electromagnetic signal propagating within the subwavelength composite portion is large compared to the characteristic feature size thereof, the structure can be treated as an overall homogeneous material. This condition is generally met when the characteristic feature size of the subwavelength pattern (typically the periodicity of the corrugations) is less than half the wavelength of the electromagnetic signal propagating therein.

In the illustrated embodiment of FIG. 12A, the subwavelength pattern is a one-dimensional corrugated grating including a series of corrugations 146 a formed from the lower waveguiding structure 122. The series of corrugations 146 a is distributed along the waveguiding axis 108 and transverse thereto. The series of corrugations 146 a includes a first section 148 a and a second section 148 b respectively proximate the first end 126 a and the second end 126 b of the lower waveguiding structure 122. In the first section 148 a, the corrugations 146 a are formed by progressively removing more and more material from the lower waveguiding structure 122 in order that the effective refractive index of the propagating mode gradually decreases toward the refractive index of the surrounding medium. Increasingly removing material from each side of the lower waveguiding structure 122 along the waveguiding axis 108 may be performed up a point where the minimum width achievable by the fabrication process used is reached. From this point, the second section 148 b begins in which material from the lower waveguiding structure is removed over its entire cross-section. Hence, in the second section 148 b, the corrugations 146 a are interleaved with gaps 146 b where portions of the lower waveguiding structure 122 are absent.

It will be understood that as long as the period of the subwavelength pattern remains smaller than half the effective wavelength of the mode propagating in the subwavelength composite portion 144, the subwavelength composite portion 144 of the lower waveguiding structure 122 can act as a homogeneous medium with an effective refractive index whose value is between those of the material forming the corrugations 146 a and the surrounding material. Such an approach may be used for any taper whose cross-section is to be tapered down to very small values. It will also be understood that the parameters of the subwavelength pattern such as its period or lack thereof, its duty cycle (i.e., the ratio of the corrugation length to the period of the subwavelength pattern) and the like may be tailored to reduce optical losses, back reflections and other adverse effects that could occur at the junction between the first and second sections 148 a, 148 b.

In the configurations describe above, the lower waveguiding structure of the spot-size converter consists of a single waveguide element. However, in other configurations, the lower waveguiding structure can include a plurality of high-index elements lying in a common horizontal plane (e.g., deposited on an SOI insulation layer).

For example, referring to FIG. 13, the lower waveguiding structure 122 includes three coplanar silicon high-index elements 150 a to 150 c extending substantially along the waveguiding axis. The high-index elements 150 a to 150 c include a central high-index element 150 b directly coupled to the first waveguide 102 at the first end 126 a of the lower waveguiding structure 122, and two narrower outer high-index elements 150 a, 150 c laterally disposed on each side of the central high-index element 150 b. Both the central element 150 b and the outer elements 150 a, 150 c can be tapered along the waveguiding axis 108 to enable vertical evanescent coupling with an upper waveguiding structure (not shown in FIG. 13), which could be embodied by the multi-rod structure of FIGS. 3 to 6 or another structure. The high-index elements 150 a to 150 c can be positioned and their effective refractive indices selected so as to be optically coupled together so that they can be viewed as a single overall waveguiding structure that can support the propagation of only few modes or even be single-mode. Of course, the number, size, shape, arrangement and material composition of the plurality of high-index elements 150 a to 150 c illustrated in FIG. 13 are provided by way of example only, and various other configurations can be envisioned in other embodiments.

It will be understood that providing a lower waveguiding structure 122 with a plurality of co-extensive high-index elements 150 a to 150 c can enlarge the horizontal extent of the propagating mode compared to a case where only a single waveguide element is provided. Also, in order to provide a smooth transition of light between the first waveguide 102 and the central element 150 b of the lower waveguiding structure 122, the optical coupling between central element 150 b and each of the outer elements 150 a, 150 c may be reduced near the first waveguide 102. For this purpose, the outer elements 150 a, 150 c in FIG. 13 are formed to gradually diverge away from the central element 150 b near the first end 126 a of the lower waveguiding structure 122.

It is to be noted that a lower waveguiding structure made of multiple elongated waveguide elements such as illustrated in FIG. 13 generally behave quite differently from longitudinal subwavelength gratings such as illustrated FIGS. 12A and 12B. In longitudinal subwavelength gratings, the light must fully pass through each section and the mode propagating through the structure has an effective index roughly determined by the duty cycle of the grating. Of course, the period of the grating should be small enough to avoid the behavior of a Bragg grating. However, in a structure composed of elongated waveguide elements, the light travels mainly in the surrounding medium and the effective index is significantly lower. Furthermore, while the elongated waveguide members are generally close enough to allow a sufficient coupling therebetween and to behave as a single guiding structure, their spacing need not fulfill a subwavelength condition.

Turning now to FIG. 9, in another embodiment of the spot-size converter 100, the plurality of high-index elements 132 of the upper waveguiding structure 124 can consist of a stack of vertically spaced high-index sheets. As for the multi-rod array describe above in reference to FIGS. 3 to 6, the multi-sheet upper waveguiding structure 124 of FIG. 9 can form an overall single-mode waveguiding structure capable of carrying a single mode whose size and effective refractive index can match those of an optical fiber. It will be understood that the effective refractive index of the upper waveguiding structure 124 can be tailored by adjusting one or more of the number, width, thickness, longitudinal and cross-sectional profiles, vertical separation and material composition of the high-index sheets.

For example, referring to FIG. 14A, there is illustrated an end view of an upper waveguiding structure provided as a stack of four vertically spaced high-index sheets made of a material having a refractive index of 1.58, such as SU-8 polymer and embedded in a low-index surrounding medium (e.g., silica). The upper waveguiding structure in FIG. 14A supports a single TE mode and a single TM mode, respectively shown in FIGS. 14B and 14C. The overlap with the fundamental mode of a SMF-28® single-mode optical fiber (Corning Incorporated), which is shown in FIG. 2, is larger than 99% at 1550 nm. It is to be noted that the upper waveguiding structure depicted in FIG. 14A can be quite robust to variations in sheet thicknesses and similarity. For example, variations as high as 25% have been found to have only a minor impact on the mode profile.

Turning now to FIGS. 15A to 15C, in accordance with another variant, the lower and upper waveguiding structures 122, 124 of the spot-size converter 100 are evanescently coupled to each other in the vertical direction and respectively consist of three coplanar tapered silicon waveguide elements 150 a to 150 c and a stack of four vertically spaced high-index sheets 132 a to 132 d. The lower and upper waveguiding structures 122, 124 are provided in a respective low-index surrounding cladding 138 a, 138 b. Hence, in the illustrated embodiment, the medium surrounding the lower and upper waveguiding structures 122, 124 is made of at least three different layers, namely an insulating layer 116, a first cladding 138 a and a second cladding 138 b (the second cladding 138 b may itself be made of more than one sublayers). As mentioned above with reference to FIG. 8A, these three layers are typically made of pure silica, but their refraction indices may differ due to the use of different deposition techniques.

The multi-sheet upper waveguiding structure 124 can have a uniform cross-section profile and be provided with an effective index that is slightly higher than that of the surrounding cladding 138 b. When the medium 138 b surrounding the multi-sheet upper waveguiding structure 124 is made of the same material as the medium 138 a surrounding the tapered waveguide elements 150 a to 150 c (e.g., silica), there will be a point along the waveguiding axis of the spot-size converter where the effective indices of the individual modes will cross, since the effective index of the lower waveguiding structure will vary from a high value (e.g., larger than 2 for the TE mode or larger than 1.7 for the TM mode) down to the value close to that of the surrounding cladding 138 a.

It will be understood that the lower waveguiding structure provided below the multi-sheet upper waveguiding structure can be composed of many lateral high-index elements or of a single waveguide element as long as the interaction between the lower and upper waveguiding structures is sufficiently strong. It will also be understood that compared to the multi-sheet approach of FIGS. 15A to 15C, the multi-rod approach of FIGS. 3 to 6 can allow for the density of high-index material in the upper waveguiding structure to be further reduced. As a result, the effective refractive index of the upper waveguide structure can be made closer to that of the surrounding medium, thereby improving the overall light distribution within the upper waveguiding structure.

The embodiment of 15A to 15C provides an example of an efficient adiabatic evanescent transfer of the TE mode between a multi-sheet upper waveguiding structure and a longitudinally tapered silicon-based lower waveguiding structure. In implementation requiring that such a transfer be efficient for both the TE and TM modes, it can be desirable that the TE and TM modes have similar effective refractive indices. The multi-sheet upper waveguiding structure discussed above is uniform and weakly guiding. Accordingly, the effective indices of both TE and TM modes are generally similar and only slightly higher than the index of refraction of the medium surrounding the sheets.

In contrast, a tapered silicon-based lower waveguiding structure such as shown in FIGS. 3 to 6 and 15A to 15C is generally characterized by spatially-dependent effective refractive indices that can be quite different for the TE and TM modes. For example, FIGS. 17A to 17C depict the TE and TM effective refractive indices around 1550 nm (TE mode: solid curve; TM mode: dashed curve) of tapers tapering from 300 nm to 50 nm in width and having different uniform thicknesses (FIG. 17A: 150 nm; FIG. 17B: 180 nm; and FIG. 17A: 220 nm). It is seen that both the TE and TM modes may have index crossing similar to what is depicted in FIGS. 7 and 7A, but that the transfer efficiency would be different. In some embodiments, it may be advantageous to have a better similarity between the TE and TM modes (e.g., in terms of their value or their slope as a function of the taper width).

Referring to FIG. 18A, there is shown another possible configuration for the lower waveguiding structure 122 of the spot-size converter. In this configuration, the lower waveguiding structure 122 is provided as a single two-level taper, which includes a bottom layer 152 a and a top layer 152 b superposed on and coextensive with the bottom layer 152 a. The bottom layer 152 a extends along the waveguiding axis 108 from the first to the second end 126 a, 126 b of the lower waveguiding structure 122, while the top layer extends from the first end 126 a to an end tip 154 located between the first and second ends 126 a, 126 b. The lower waveguiding structure 122 has a first height h₁ corresponding to the bottom layer 152 a and a second height h₂ corresponding to a superposition of the bottom and top layers 152 a, 152 b.

It will be understood that if the geometrical dimensions of the top and bottom layers 152 a, 152 b are properly designed, the cross-section of the lower waveguiding structure 122 can have a square profile, and thus equal TE and TM effective refractive indices, apart from a slight impact due to other material such as the substrate that can slightly break the structure symmetry, at two positions 156 a, 156 b along the waveguiding axis 108. To achieve this, a width w₁ of the lower waveguiding structure 122 at the end tip 154 of the top layer 152 b should be equal to the first height h₁ (position 156 a), while a width w₂ of the lower waveguiding structure 122 at the first end 126 a thereof should be equal to the second height h₂ (position 156 b).

Referring to FIG. 19, it can be seen that the TE and TM effective refractive indices of the lower waveguiding structure of FIG. 18A exhibit a higher degree of similarity along the waveguiding axis (TE mode: solid curve; TM mode: dashed curve). For example, at position 156 a, the lower waveguiding structure 122 of FIG. 18A may have a 150 nm×150 nm square cross-section and a low effective refractive index (i.e., close to that of the surrounding medium) and substantially equal for the TE and TM modes. Likewise, at position 156 b, the lower waveguiding structure 122 of FIG. 18A may have a 220 nm×220 nm square cross-section and a high effective refractive index (i.e., about 1.56 according to FIG. 19), again substantially equal for the TE and TM modes. It is to be noted that an intermediate waveguiding element may be provided between the first end 126 a of the two-level taper and the first waveguide if the dimensions of the first waveguide differ from those at the first end of the two-lever taper. It will be appreciated by one skilled in the art that engineering the lower waveguiding structure such as in FIG. 19 may reduce polarization-dependent losses (PDL) in some embodiments of the spot-size converter.

It will be understood that, in other implementations, the lower waveguiding structure may be embodied by a two-level taper that is not configured to have a square cross-section at any point along its length. Likewise, in yet other implementations, the lower waveguiding structure 122 may be embodied by a two-level taper in which either or both of the top and bottom layers 152 a, 152 b are provided with a subwavelength composite portion 144 that defines a subwavelength pattern, as illustrated in FIGS. 18B and 18C.

It is to be noted that the single-level tapered lower waveguiding structure 122 using subwavelength gratings introduced briefly above and illustrated in FIG. 12B could be used as an alternative to the two-level tapered lower waveguiding structure of FIG. 18A. In FIG. 12B, at the junction between the first and second sections 148 a, 148 b, the lower waveguiding structure 122 is a 220 nm×220 nm waveguide having a subwavelength grating with a 50% duty cycle. At this point, the effective index is found to be relatively low (i.e., close to the surrounding medium) and equal for TE and TM modes since the waveguide has a square shape. Meanwhile, at the first end 126 a, the lower waveguiding structure 122 is again a 220 nm×220 nm waveguide, resulting in a larger effective index. The structure of in FIG. 12B is thus found to act very similarly to the structure shown in FIG. 18A. Again, intermediate waveguiding element may be provided between the first end 126 a of the two-level taper and the first waveguide if the dimensions of the first waveguide differ from those at the first end of the two-lever taper.

As discussed previously, the upper waveguiding structure of the spot-size converters described herein can be designed to match an optical fiber that would be optically coupled using a thin adhesive layer 158 such as illustrated in FIG. 20 or through free space via a lens or another suitable optical component.

In other implementations, the upper waveguiding structure of the spot-size converters described herein could alternatively be designed to allow an evanescent coupling from the top as illustrated in FIG. 21. In such implementations, the role of the upper waveguiding structure 124 is to increase the mode size to facilitate the evanescent coupling with a larger second waveguide 104 such as a polished fiber. Having a larger mode size allows a better compatibility with the use of an adhesive layer 158 between the chip in which the spot-size converter is implemented and the polished fiber. Such an adhesive layer could have a thickness in the range of 1 to 3 μm.

Referring now to FIG. 22, in some embodiments, the high-index elements 132 (e.g., the stack of vertically spaced high-index sheets) forming the upper waveguiding structure 124 can have a cross-section that varies longitudinally (e.g., in width). Such a design thus varies the effective refractive index of the upper waveguide structure along its length and can provide an effective index crossing point with the mode of the optical fiber. It is noted that, in FIG. 22, the rows of high-index sheets are slightly shifted laterally from one another to better illustrate the configuration of the upper waveguiding structure.

Referring now to FIGS. 29A and 29B, it will be understood that in embodiments where the upper waveguiding structure 24 of the spot-size converter 100 is evanescently coupled at its first end 128 a to the second waveguide 104 (e.g., to an optical fiber) and toward its second end 128 b to the lower waveguiding structure 122, it can be advantageous that the upper waveguiding structure 124 includes two end portions 160 a, 160 b of different cross-sectional profiles. The coupling first section 160 a would be designed for optimal coupling (e.g., butt coupling) to the second waveguide 104, while the second end portion 160 b would be designed for optimal evanescent coupling to the lower waveguiding structure 122. In some of these embodiments, the upper waveguiding structure 124 could further include an intermediate tapered portion 162 extending between and optically linking the first and second end portions 160 a, 160 b. In some implementations, the intermediate tapered portion 162 may be adiabatically tapered. In FIGS. 29A and 29B, the high-index elements 132 forming the upper waveguiding structure 124 are embodied by a two-dimensional array of “rods” where each “rod” includes two length segments of different cross-sections connected to each other by a tapered segment. It is to be noted that this concept is applied to high-index sheets in FIG. 22.

Referring to FIG. 23, an optical fiber polished at a blazing angle could be used instead of a fiber polished from the side.

The upper waveguiding structure of the spot-size converter can be varied not only longitudinally, but also in terms of the number, widths and relative positioning of the multiple vertically spaced rows (e.g., rows of high-index rods as in FIGS. 3 to 6 or high-index sheets as in FIGS. 15A to 15C). Such variations can be done to provide a mode of further increased size at the chip edge.

Referring to FIGS. 24A and 24B, there is illustrated an embodiment of the spot-size converter where the rows of high-index elements 132 (e.g., high-index sheets) of the upper waveguiding structure 124 are staggered along the waveguiding axis 108 (in FIG. 24B, the sheets are slightly shifted laterally to better illustrate the configuration of the upper waveguiding structure). The vertical position of the high-index elements 132 can be varied along their length, for example by providing a region of the cladding structure etched down close to the lower waveguiding structure 122 only close to the coupling region 140 where evanescent coupling between the lower and upper waveguiding structures 122, 124 occurs. Such an arrangement could serve to minimize the evanescent coupling to the substrate 114. Etching the top cladding could then be done such that the etching edges are not vertical but are at an angle, as this would provide a smooth curvature of the upper waveguiding structure 124 toward the lower waveguiding structure 122. Furthermore, while in the embodiment of FIGS. 24A and 24B, some of the high-index sheets are stopped abruptly, they could alternatively be provided with a width taper to smooth the transition such as illustrated in FIG. 25. Also, a subwavelength approach such as depicted in FIGS. 12A and 12B could be employed.

It will be understood that the rows of high-index elements of the upper waveguiding structure may be viewed as a distribution of different materials in a certain region. This distribution of different materials can be designed to be equivalent to a uniform homogeneous material from the perspective of its waveguiding property. As a result, the rows of high-index elements could be arranged such that they would form an upper waveguiding structure exhibiting a curvature allowing light to emerge out of the chip. In that case, an optical fiber located in the vicinity of the chip could collect the emerging light. Of course, injecting the light from the fiber to the chip could be performed as well. Such an embodiment is illustrated in FIG. 26.

When the lower waveguiding structure is embodied by a single longitudinally tapered silicon waveguide such as in FIGS. 3 to 6, it can be designed for providing a good transfer predominantly to either the TE mode or the TM mode. Referring to FIG. 27, in some embodiments, the lower waveguiding structure 122 could include two distinct and uncoupled laterally spaced tapered waveguide elements 164 a, 164 b, having different effective refractive indices (due for example to a difference in height), placed under the rows of the high-index elements 132 forming the upper waveguiding structure 124. The two tapered waveguide elements 164 a, 164 b would be designed differently so that one waveguide element 164 a is configured to preferably carry the TE mode, while the other waveguide element 164 b is configured to preferably carry the TM mode. In such a configuration, TE and TM polarized modes injected into the upper waveguiding structure 124 from the second waveguide would be evanescently coupled into the two laterally spaced tapered waveguide elements 164 a, 164 b according to their respective polarization. In particular, it is to be noted that in this configuration, each of the TE and TM modes will be predominantly transferred into its corresponding tapered waveguide element even if they could have been transferred to the tapered waveguide element corresponding to the other polarization. In other words, each of the TE and TM modes is coupled to its corresponding tapered waveguide element before the other tapered waveguide element could have the possibility of doing so.

Advantageously, the spot-size converters described herein can provide an in-line configuration through vertical evanescent coupling which can be compatible with CMOS technology. In some embodiments, the spot-size converter can provide an efficient coupling of light between a submicron silicon waveguide and another waveguide carrying a significantly large mode (e.g., an optical fiber with an MFD of up to 10 μm) while remaining substantially single-mode. For example, in some embodiments of the spot-size converter, the coupling from a submicron silicon waveguide to an HNA optical fiber having an MFD of 6.6 μm can exhibit an efficiency as high 94% (0.27 dB loss), leading to a total coupling loss of 0.37 dB between the silicon waveguide and a SMF-28® optical fiber (Corning Incorporated) when taking into account an additional splice loss of 0.1 dB between the 6.6 um MFD HNA fiber and the SMF-28® optical fiber. Furthermore, in some embodiments, insertion losses ranging from 0.6 to 0.8 dB for the TE mode and from 1.0 to 1.7 dB for the TM mode have been measured experimentally over the C-band including the 6.6 μm MFD HNA fiber to SMF-28 fusion splice loss, as illustrated in FIG. 30.

It is also to be noted that some embodiments of the spot-size converter have good tolerance against fabricated-induced variations in the positioning, size and shape of the waveguide elements forming the upper waveguiding structure, as well as against vertical, horizontal and longitudinal misalignments in the positioning of an optical fiber coupled at one end of the spot-size. A 1-dB bandwidth misalignment tolerance of ±1.7 μm between the optical fiber and the spot-size converter in the plane transverse to the waveguiding axis (i.e., along the width and height dimensions) and greater than 20 μm along the waveguiding axis was measured. Further advantageously, in some embodiments, the spot-size converter can be designed such that the TE and TM modes have similar effective refractive indices, especially in the coupling region where the lower and upper waveguiding structures are evanescently coupled to each other, so to obtain a polarization-independent operation.

Of course, numerous modifications could be made to the embodiments described above without departing from the scope of the present invention. 

1. A spot-size converter for coupling light between a first waveguide and a second waveguide respectively supporting a first and a second propagation mode having substantially different dimensions, the spot-size converter extending along a longitudinal waveguiding axis and comprising: a lower waveguiding structure having a first and a second end opposed to the first end, the first end being coupled to the first waveguide to receive light therefrom or transmit light thereto in the first propagation mode, the lower waveguiding structure being characterized by an effective refractive index; and an upper waveguiding structure having a first and a second end opposed to the first end, the first end being coupled to the second waveguide to transmit light thereto or receive light therefrom in the second propagation mode, the upper waveguiding structure being characterized by a second effective refractive index and comprising a plurality of high-index elements extending substantially along the waveguiding axis and arranged in multiple vertically spaced rows, at least part of the upper waveguiding structure extending at least partially over the lower waveguiding structure so as to define a coupling region wherein a crossing of the first and second effective refractive indices occurs, thereby enabling evanescent coupling of light between the lower and upper waveguiding structures.
 2. The spot-size converter according to claim 1, wherein at least one of the lower and upper waveguiding structures tapers longitudinally in the coupling region.
 3. The spot-size converter according to claim 1, wherein at least one of the plurality of high-index elements of the upper waveguiding structure tapers longitudinally in the coupling region.
 4. The spot-size converter according to claim 1, wherein the lower waveguiding structure comprises a plurality of high-index elements lying in a common horizontal plane.
 5. The spot-size converter according to claim 1, wherein the lower waveguiding structure consists of a single waveguide element extending along the longitudinal waveguiding axis.
 6. The spot-size converter according to claim 5, wherein the single waveguide element comprises a longitudinally tapered portion tapering down toward the second end.
 7. The spot-size converter according to claim 6, wherein the longitudinally tapered portion forms an adiabatic taper.
 8. The spot-size converter according to claim 5, wherein the single waveguide comprises a subwavelength composite portion that defines a subwavelength pattern having a characteristic size which is less than half an effective wavelength of light when propagating therein.
 9. The spot-size converter according to claim 5, wherein the single waveguide element has a bottom layer extending along the waveguiding axis from the first to the second end and a top layer superposed on and coextensive with the bottom layer, the top layer extending from the first end to an end tip located between the first and second ends.
 10. The spot-size converter according to claim 9, wherein at least one of the top layer and the bottom layer of the single waveguide element comprises a subwavelength composite portion that defines a subwavelength pattern having a characteristic size which is less than half an effective wavelength of light when propagating therein.
 11. The spot-size converter according to claim 9, wherein at least one of the top and bottom layers comprises a longitudinally tapered portion tapering down toward the second end.
 12. The spot-size converter according to claim 9, wherein the single waveguide element has a first height corresponding to the bottom layer and a second height corresponding to a superposition of the bottom and top layers, a width of the single waveguide element at the first end being equal to the second height and the width of the single waveguide element at the end tip of the top layer being equal to the first height.
 13. The spot-size converter according to claim 1, wherein the lower waveguiding structure is part of a silicon-on-insulator (SOI) arrangement comprising a silicon substrate, an insulating layer formed on the silicon substrate, and a silicon layer formed on the insulating layer and patterned to form the lower waveguiding structure.
 14. The spot-size converter according to claim 1, wherein the plurality of high-index elements of the upper waveguiding structure consists of a stack of vertically spaced high-index sheets.
 15. The spot-size converter according to claim 1, wherein the plurality of high-index elements of the upper waveguiding structure consists of a plurality of high-index rods arranged in a two-dimensional array in a plane transverse to the waveguiding axis.
 16. The spot-size converter according to claim 1, wherein each of the lower and upper waveguiding structures is a single-mode waveguiding structure.
 17. The spot-size converter according to claim 1, wherein the lower and upper waveguiding structures are made of silicon, silicon nitride, silicon carbide, silicon oxynitride, silicon oxide, indium phosphide, gallium arsenide, a polymer or a combination thereof.
 18. The spot-size converter according to claim 1, wherein a lowest-lying row of the plurality of high-index elements lies in a same horizontal plane and is made of a same material as the lower waveguiding structure.
 19. A spot-size converter for coupling light between a first waveguide and a second waveguide respectively supporting a first and a second propagation mode having substantially different dimensions, the spot-size converter extending along a longitudinal waveguiding axis and comprising: a single lower waveguide element having a first and a second end opposed to the first end, the first end being coupled to the first waveguide to receive light therefrom or transmit light thereto in the first propagation mode, the lower waveguide element being characterized by a longitudinally varying first effective refractive index that decreases toward the second end; and an upper waveguiding structure having a first and a second end opposed to the first end, the first end being coupled to the second waveguide to transmit light thereto or receive light therefrom in the second propagation mode, the upper waveguiding structure being characterized by a second effective refractive index and comprising a plurality of high-index elements extending substantially along the waveguiding axis and forming an array of rows and columns in a plane transverse to the waveguiding axis, at least part of the upper waveguiding structure extending at least partially over the lower waveguide element so as to define a coupling region wherein a crossing of the first and second effective refractive indices occurs, thereby enabling evanescent coupling of light between the lower waveguide element and the upper waveguiding structure.
 20. The spot-size converter according to claim 19, wherein the lower waveguide element comprises a longitudinally tapered portion tapering down toward the second end.
 21. A silicon photonic spot-size converter for coupling light between a first waveguide and a second waveguide respectively supporting a first and a second propagation mode having substantially different dimensions, the spot-size converter extending along a longitudinal waveguiding axis and comprising: a silicon substrate; an insulating layer formed on the silicon substrate; a silicon core formed on the insulating layer, the silicon core having a first end and a second end opposed to the first end, the first end being coupled to the first waveguide to receive light therefrom or transmit light thereto in the first propagation mode, the silicon core being characterized by a longitudinally varying first effective refractive index that decreases toward the second end; and a plurality of high-index rods extending substantially along the waveguiding axis and forming a array of rows and columns in a plane transverse to the waveguiding axis, each high-index rod having a first end and a second end opposed to the first end, the first end being coupled to the second waveguide to transmit light thereto or receive light therefrom in the second propagation mode, the plurality of high-index rods being collectively characterized by a second effective refractive index, at least part of the upper waveguiding structure extending at least partially over the silicon core so as to define a coupling region wherein a crossing of the first and second effective refractive indices occurs, thereby enabling evanescent coupling of light between the silicon core and the plurality of high-index rods.
 22. The silicon photonic spot-size converter according to claim 21, wherein the silicon comprises a longitudinally tapered portion tapering down toward the second end.
 23. The silicon photonic spot-size converter according to claim 21, further comprising a cladding structure embedding both the silicon core and the plurality of high-index rods.
 24. The silicon photonic spot-size converter according to claim 23, wherein the cladding structure comprises: a first cladding formed on the insulating layer and surrounding the silicon core; and a second cladding formed on the first cladding and embedding the plurality of high-index rods. 